Typical ABS systems for vehicles include an electric motor driven opposed piston-type pump for supplying pressurized brake fluid to any one of the vehicle brakes on instant, automatically controlled, demand. The electric motor is a part of the ABS unit and includes a drive shaft having an eccentric or cam portion. A pair of pumping pistons are located on opposite sides of the drive shaft within a body of the pump. Respective input and output passageways allow for proper circulation of fluid through the pump. The output associated with each pumping piston provides high pressure brake fluid, approximately 2500 psi, to an associated brake or group of brakes on the vehicle. The ABS system is typically integrated into the vehicle braking system and monitors specific braking parameters such as vehicle acceleration, vehicle speed, the coefficient of friction of the road, and the amount of brake pedal pressure input from a driver. Once minimum threshold values for the ABS characteristics are exceeded, a controller activates the ABS system and the ABS pump provides pressurized brake fluid as needed, instead of pressurization from the brake pedal and master cylinder.
Conventional ABS pumps have complex piston structures including through bores drilled for providing passageways for brake fluid to pass through the piston. To control direction of flow, one-way inlet check valves are provided in the piston fluid passageway between the inlet and the outlet passageways of the pump. The one-way check valves allow fluid from the inlet to pass through the piston to the outlet but prevent flow in the opposite direction. Thus, the piston is able to build up pressure for delivery of pressurized fluid to the output. Conventional pistons have O-rings or D-rings for sealing engagement between the exterior of the piston and the associated housing bore. One-way check valves usually have a ball biased by a spring and require a machined seating surface for the ball to properly seal a desired passage. In addition, sleeves and connectors are utilized for retaining the spring and ball within the piston. The check valve increases the overall length of the piston and thereby requires the pump to be proportionally larger as well. Therefore, the overall weight of the pump is undesirably heavier.